In general, a bill processing apparatus, which handles a bill as one of the embodiments of the paper sheet, is incorporated into a service device such as a game medium rental machine installed in a game hall, a vending machine or a ticket-vending machine installed in a public space, or the like which identifies the authenticity of the bill inserted from a bill insertion slot by a user and provides various types of products and services in accordance with a value of the bill having been judged as authentic.
The above-mentioned authenticity of the bill is judged, for example, by an authenticity judging device comprising a light emitting part irradiating the bill moving in a bill traveling route with light and a light receiving sensor receiving transmitted light and reflected light of the light irradiated by the light emitting part, and an authenticity judgment process is conducted by comparing received light data transmitted from the light receiving sensor with legitimate data as disclosed in Patent Reference 1.
Meanwhile, it is known that the above-mentioned light receiving sensor of the authenticity judging device has a device property to deteriorate with the lapse of time. As such deterioration is caused with the lapse of time, an output value is lowered such that received light data of the bill may represent dim light. Therefore, so-called black calibration (black balance) to correct a reference value of the output for the lowest brightness is performed every predetermined period of time. For example, it is typically performed that an output from the light receiving sensor is read in a state where no light is irradiated from the light receiving part when the bill processing apparatus is turned on and the received light data having been read in this way is set as the reference value for the lowest brightness.    [Patent Reference 1] Japanese unexamined patent application publication No. 2001-279487